1. Field of the Invention
The present invention relates to a microwave oven which heats food with microwaves for cooking and more particularly to a microwave oven waveguide where electric output and electric field distribution at a cavity are kept constant regardless of food load by minimizing change in impedance of the waveguide depending on food load for cooking.
2. Description of the Prior Art
Generally, a microwave oven is designed to radiate microwaves generated from a magnetron through a waveguide for heating food placed at a cavity to be dielectric heated for cooking.
FIG. 1 is a brief sectional view of a waveguide of a microwave oven in accordance with a first embodiment of the conventional art, and FIG. 2 is a structure analysis drawing of a waveguide shown in FIG. 1. One side of the waveguide(1) includes a magnetron insertion hole(9) while the other side thereof includes a rectangular shape of opening(7) for radiating the microwaves generated from the magnetron(3) into the cavity.
The microwaves generated from the magnetron(3) are radiated inwards through the waveguide(1) for heating food at the cavity(5) to be dieletric heated.
Here, as shown in FIG. 2', if power from the magnetron(3) is P.sub.in, and if electric output to a specific position of the cavity(5) is P.sub.out, then, P.sub.out is expressed in the following mathematical formula. EQU P.sub.in =E.sup.2.sub.s Formula 1 EQU E.sub.y =E.sub.s sin (x) Formula 2 EQU P.sub.out =(E.sub.y).sup.2 =(E.sub.s sin (x)).sup.2 =E.sup.2.sub.s sin (x).sup.2 Formula 3
At the first through third mathematical formulas, E.sub.s is electric field energy formed by the microwave generated from the magnetron(3), namely input electric field energy, E.sub.y is electric field energy formed at the specific position of the cavity(5), namely the output electric field energy.
The output of the magnetron(3) is obtained by squaring the electric field power, E.sub.s, formed by the microwaves generated therefrom. As the microwaves generated from the magnetron(3) is a specific phase, namely a sine wave, the electric field energy at the specific position of the cavity, E.sub.y, is obtained by multiplying the sine value, sin (x), to the electric field energy formed by the microwaves, E.sub.s, and the output at a the specific position of the cavity, P.sub.out, is obtained by squaring the electric field energy, E.sub.y.
Therefore, the output at the specific position of the cavity, P.sub.out is formed as results from multiplying the sine value, sin (x), to the output from the magnetron, P.sub.in, wherein the sine value, sin (x) or the phase, is changed according to load of food to be cooked, thereby changing the output at the specific position of the cavity(5), P.sub.out.
The characteristic impedance of the waveguide according to the load change of food is described in a polar chart, as shown in FIG. 3. FIG. 3 illustrates the characteristic impedance according to the water load of 2000, 1000, 500 and 100 cc at a microwave frequency range of 2.44-2.47 GHz.
As shown in FIG. 3, in case that the water load is 2000 cc, the impedance of waveguide, voltage standing wave ratio(VSWR), is low. On the other hand, in case that the water load is 100 cc, the impedance of waveguide, voltage standing wave ratio(VSWR) is so high that output from the microwave oven is of small quantity.
Though the output from the microwave oven is somewhat high in case of large food load, there is a problem in that the impedance of the waveguide is increased leading to low output from the microwave oven in case of small food load.
In addition, there is another problem in that the electric field distribution at the cavity is not kept constant because the change in the impedance of the waveguide becomes big according to the change in food load to be cooked.
Furthermore, even if the impedance of the waveguide is to be matched to that of the cavity to improve the output from the microwave oven, the aforementioned structure of the waveguide is not designed to get the impedance thereof and that of a specific cavity matched. Therefore, there is further problem in that one waveguide can not be adapted to a variety of cavities, so that each waveguide is to be designed for respective cavity.
On the other hand, a waveguide of a microwave oven disclosed at a Japanese laid-open patent No. Hei 6-111933 is developed to improve the equalized heating efficiency at food of a cavity thereof, and to shorten the waveguide for making an easy arrangement of electric parts therein.
As shown in FIG. 4, the waveguide is provided with one pair of wave supply holes(11a and 11b) at one side wall, a cavity(12) to get food to be cooked placed in, a magnetron(14) disposed between the wave supply holes(11a and 11b) apart from the lateral wall having the wave supply holes(11a and 11b) to generate the microwave of .lambda..sub.g frequency, a waveguide being at .lambda..sub.g /4 distant from an antenna(13), having a separating plane in parallel to the antenna(13), covering the wave supply holes(11a and 11b), supporting the magnetron(14) and guiding the microwave passed through the wave supply holes(11a and 11b) to the cavity(12).
In case of the waveguide of the microwave oven described above, the wave generated from the magnetron(14) forms at the waveguide the voltage standing wave which is radiated into the cavity through the wave supply holes(11a and 11b) for equally heating the food therein.
However, in the conventional waveguide of the microwave oven a pair of wave supply holes(11a and 11b) are formed at upper portion of one lateral wall of the cavity(12), and microwaves generated from the magnetron(14) are radiated through the wave supply holes(11a and 11b). Therefore, even if the waveguide made a contribution to improving in equalized heating efficiency of food owing to a better radiating function of the microwaves, there is a problem in that the waveguide is not properly adapted to the change of the output from the microwave oven according to the food load.